Causal Agent:
Alternaria alternata f. sp. lycopersici

Distribution:
USA (California)

Symptoms:
Symptoms may occur on all above ground parts of the plant. Dark brown cankers with concentric rings form on stems and are often associated with wound sites. These cankers may enlarge, eventually girdling the stem and killing the plant. A brown, dry rot develops and brown streaks can extend into the pith above and below the cankers. A toxin produced by the fungus growing in the stem canker moves into the upper part of the plant, killing the interveinal leaf tissue. As the disease progresses, curling of leaf margins and eventual death of the leaf occurs. Fruit symptoms initially appear as small, gray flecks which later enlarge and become dark and sunken with the characteristic concentric rings. Symptoms may not be present on mature green fruit, however, they can develop rapidly in transit.

Conditions for Disease Development:
The fungus can survive in the soil and crop debris for more than a year. Rain, overhead irrigation and dews favor development of this disease, and its conidia are easily spread by the wind. Stem wounds created by pruning allow entry of the fungus, however, infection can also occur without wounds.

Control:
It is difficult to control this disease with fungicides so resistant varieties should be used.

Typical black stem lesions.              Close-up of the stem canker. 

Toxin induced leaf interveinal           Dark sunken fruit lesions.  

chlorosis and necrosis..